This application claims the priority of German application 197 52 772.8, filed Nov. 28, 1997, the disclosure(s) of which is (are) expressly incorporated by reference herein.
The invention relates to an air gap insulated exhaust pipe with branch pipe stub and method of manufacturing same.
A method for manufacturing an exhaust pipe is known from German patent document DE 195 11 514 C1. This document teaches the manufacture of an exhaust pipe, insulated by an air gap, and provided with a branch stub in conjunction with a combination of several exhaust pipes to form an exhaust manifold with the outer jacket of the branched exhaust pipe consisting of two half-shells connected with one another being a common component of all the exhaust pipes of the exhaust manifold. Thus, the inner tubes of the exhaust pipes are initially pushed onto one another with a push fit and provided in a costly fashion with special spacing rings which later evaporate after assembly during the operation of the exhaust system. The plug connection is then inserted into a lower shell of the outer jacket and positioned in an awkward fashion. Since the individual tubes which are subject to manufacturing tolerances are displaceable with respect to one another and have different insertion lengths from one plug-in connection to the next plug-in connection because of the assembly work, and the spacing rings are themselves subject to manufacturing tolerances and also, because of their design relative to the shape of the lower shells, rarely abut the latter circumferentially, the manufacture of the entire exhaust manifold is subject to tolerances due to these factors alone. There is no such thing as exact reproducibility.
It is important to observe during assembly that a certain minimum insertion length is maintained so that the individual internal tubes do not slide apart. This retention requires visual estimation and hence considerable effort. During the transfer of the parts to the welding station, vibrations and centrifugal forces can likewise occur that can lead to additional displacement of the individual inner tubes with respect to one another and with respect to the lower shell of the outer jacket, which can lead to the plug-in connection coming apart. The transposition of the troublesome positioning of the inner tubes in the lower shell of the outer jacket by means of the spacing rings and the tolerances resulting from manufacturing technology in the design of the inner tubes as well as the different associated relative positions of the inner tube inside and outside the outer jacket to the outer jacket, an individual branched inner tube with an outer jacket consisting of two half-shells can be produced in simple fashion. The inner tube with the branched stub with the stated manufacturing tolerances is never located inside the outer jacket with the desired defined circumferential air gap.
Due to the delayed rebound of the two sheet-metal half-shells following deep drawing, the two half-shells do not abut one another continuously tightly and thus gap-free. Therefore, in the welding station, the upper shell of the outer jacket is placed on the lower shell and pressed against the latter. In this situation as well, there are vibrations of the plug-in connection and/or displacement of the relative position of the branched inner tube in the outer jacket. Finally, the shells of the outer jacket are laser-welded to one another. After the pressure is relieved, because of the nonuniformity of the contact surfaces of the half-shells, considerable tensile forces act on the welded seam, which reduces the long-term load-carrying capacity of the assembly, especially of the outer jacket, and can result in failure of the part during operation of the exhaust line.
In addition, the welding of the half-shells to form a crimped seam is relatively awkward, especially since at the transition to the cutout in the outer jacket for the branch stub of the inner tube, because of the edge radii, a triangular gore results which must be welded for processing safety, which in practice logically takes place only with the assistance of an additional material. In addition, the crimped seam can also be subjected only to limited mechanical loading due to its design. To secure the inner tube to the outer jacket, a weld is also required that forms a round seam, in other words, a circumferential hollow weld in the end area of the branch stub, with the end of the inner tube of the stub being slightly recessed relative to the opening of the outer jacket. The outer jacket is also designed to project considerably into space because of the branched exhaust pipe, which, during the manufacture of the half-shells by deep-drawing, cannot achieve branching and thus is not suitable for a defined formation of an outer jacket relative to the design of the inner tube. However, this requires considerable space and increases the weight of the branched exhaust pipe. In addition, the design of a defined, uniformly constant air gap with a branched exhaust pipe cannot be achieved by this design.
A goal of the invention is to improve on a method of manufacturing an exhaust pipe that an air-gap-insulated exhaust pipe with a branch stub can be manufactured exactly reproducibly in simple fashion, and which can easily be built up without adversely affecting the dimensional accuracy of the width of the air gap and the position of the inner tube relative to the outer jacket.
This and other goals have been achieved according to the present invention by providing a method for producing an air-gap-insulated exhaust pipe with a branch stub for a vehicle exhaust line having an inner tube with a branch for carrying exhaust surrounded at a distance by an outer jacket to form an insulating air gap, said method comprising: providing two tubes having a corresponding shape, inserting said tubes into one another with limited play to form a double tube, placing said double tube in a first internal high-pressure shaping tool having a first engraving including a branch, sealing off both ends of said double tube to be tight to a high-pressure fluid, closing the first shaping tool and introducing a pressure fluid into an interior of the inner tube of the double tube such that the double tube expands to match the contours of the first engraving to form a shaped double tube including a double-walled branch stub blown out of the double tube into the branch, relieving the pressure fluid in the first shaping tool, removing the shaped double tube from the first shaping tool, placing the shaped double tube in a second internal high-pressure shaping tool having a second engraving which holds the shaped double tube at axial end areas in a fit with play, the second engraving being spaced apart from the shaped double tube between the axial end areas including the branch stub to define an expansion chamber, closing the second shaping tool and introducing a pressure fluid between the two tubes that form the shaped double tube and simultaneously into the interior of the inner tube, such that the outer tube expands into said expansion chamber and engages said second engraving of the second shaping tool to define an insulating air gap between the outer tube and the inner tube, an end of the branch stub facing away from the rest of the double tube being externally supported without yielding via a second tool counterpunch located in a branch of said second engraving, relieving the pressure fluid in the second shaping tool, removing the finished double tube from the second shaping tool, and cutting off a cap area at the end of the branch stub to form a through opening between the interior of the inner tube and the outside of the air-gap-insulated exhaust pipe.
This and other goals have been achieved according to the present invention by providing a method for producing an air-gap-insulated exhaust pipe with a branch by internal high-pressure forming, said method comprising: placing an inner tube inside of an outer tube to form a double tube; placing said double tube in a first internal high-pressure shaping tool having a first engraving including a branch; forming an intermediate shaped double tube by introducing a pressure fluid into an interior of the inner tube such that the double tube expands into the branch; arranging the intermediate shaped double tube in a second internal high-pressure shaping tool having a second engraving which is circumferentially larger than said first engraving such that an exterior of said shaped double tube is spaced apart from said second engraving to define an expansion chamber therebetween; forming a final shaped double tube by introducing a pressure fluid between the two tubes and simultaneously into the interior of the inner tube, such that the outer tube expands into said expansion chamber into engagement with said second engraving of the second shaping tool to form an insulating air gap between the outer tube and the inner tube.
This and other goals have been achieved according to the present invention by providing a tool system for producing an air-gap-insulated exhaust pipe with a branch by internal high-pressure forming a double tube including an inner tube nested inside of an outer tube, said tool system comprising: a first internal high-pressure shaping tool having a first engraving including a main receiving area for receiving said double tube and a branch extending radially from said main receiving area for supporting a portion of the double tube to be expanded into the branch under high-pressure forming; a second internal high-pressure shaping tool having a second engraving which is circumferentially larger than said first engraving.
According to the invention, a simple manufacture of the branched exhaust pipe is possible from two welded or drawn double tubes that are pushed into one another, prefabricated by cutting or bending. There is no costly deep-drawing of the half shells that form the outer jacket or any very complicated welding of the two half-shells, especially at the transition between the crimped seam and the round seam in the vicinity of the branched stub, at which transition a gore results because of the edge radii, which can be welded shut with additional material to ensure the long-term load-carrying capacity of the welded seam. Moreover, no assembly-intensive error- and tolerance-prone assembly of the exhaust pipe is necessary, but the relative positions of the two tubes pushed into one another is determined after initial shaping by endwise clamping of the tubes to one another. No spacing rings are required because there is no insulating air gap due to the suitable assembly of the inside tube in the outside jacket; rather the gap is created automatically by the second shaping of the double tube. As a result of the established nondisplaceable relative positions of the two tubes following the first shaping relative to one another and the dependencies of the insulating air gap widths only on the shape of the engraving and the completeness of the shaping, both of which can be readily controlled, a constant gap width is ensured in simple fashion.
Since the branch stub is made of a double-walled tube consisting of an inner tube and an outer tubexe2x80x94the later outer jacketxe2x80x94during the first internal high-pressure shaping process, the outer tube adjusts as a function of the method with proper contours to the inside tube. As a result, with a branched exhaust pipe and thus with the entire exhaust line, by contrast to deep-drawn half-shells, space, material, and weight are saved. Because of the matching of the contours of the outer tube, the length of the branched stub can be made relatively short so that when the branched exhaust pipe is connected to additional exhaust line parts in the vicinity of the branched stub, a compacting process, in other words, a gain in space for this assembly, can be achieved.
In addition, the branched exhaust pipe according to the invention can withstand permanent loads better than the conventional solutions, since the exhaust pipe contains welded seams only at the connecting points to other parts of the exhaust line during the connecting process, which welded seams can be made in simple fashion in the shape of circumferential hollow welds that can withstand high mechanical loads. The crimped seams that are prone to failure in known exhaust pipes are eliminated.
Finally, as a result of the freedom from tolerance that is linked to the method of internal high-pressure shaping, exact reproducibility of the branched exhaust pipe is made possible and hence it is easier to automate exhaust pipe manufacture without finishing work for improvement. Because of the clamping of the individual tube walls at both ends of the exhaust pipe, the exhaust pipe can be readily coupled there to other parts without the relative position of the inner tube with respect to the outer tube and the gap width changing. At the end of the branch stub, the inner tube, if desired, can be pushed in simple fashion onto another inner tube of another air-gap-insulated exhaust pipe and remain there in a slide fit while the outer tubes of the two exhaust pipes are welded to one another forming a simple circumferential hollow weld. The exhaust pipe according to the invention is thus easily installed on other parts of the exhaust line because of its space advantages and its problem-free and reliable as well as rapid connection.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.